Since plastics optical materials are lightweight, hardly breakable, and excellent in dyeability as compared with optical materials made of inorganic materials such as glass, they are widely used as optical materials for eyeglass lenses, camera lenses, and the like. Recently, there has been a demand for high performance of optical materials having such properties as a high transparency, a high refractive index, a low specific gravity, a high heat resistance, and a high impact resistance.
Polythiourethanes among the optical materials are widely used as an optical material by virtue of their excellent optical properties and excellent mechanical properties. Polythiourethanes may be prepared by reacting a thiol and an isocyanate. Lenses produced from polythiourethanes are widely used since they have a high refractive index, a lightweight, and a relatively high impact resistance.
Xylylene diisocyanate among the isocyanates used as a raw material of a polythiourethane is generally synthesized from xylylenediamine by a phosgene method or a non-phosgene method and is commercially used in a very wide range of fields.
Although xylylene diisocyanate has various advantages, it also has disadvantages in that it is difficult for a product produced therefrom to have uniform characteristics because of the high reactivity of the NCO groups and that it is liable to lose its original physical properties due to side reactions when it is stored for a long period of time. Specifically, xylylene diisocyanate forms a dimer or a trimer due to the self-reaction of the NCO groups, thereby changing its NCO % value. As a result, a solid material is precipitated, and this phenomenon is further accelerated by the reaction with moisture introduced from the environment. Various stabilizers may be used to prevent or retard such problems, but this may cause yellowing or other problems in the production of polyurethanes as they affect the reactivity of xylylene diisocyanate. Furthermore, unlike other types of isocyanates, xylylene diisocyanate has a much greater side effect depending on the kind and content of additives, which makes it difficult to find an optimal formulation.